Physiology and ecology of ammonia-oxidizing Archaea

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Marine Crenarchaeota have been found to represent a major fraction (up to 40 %) of the planktonic microbial community in the Ocean. The first cultured member of the abundant marine group 1 Crenarchaeota, candidatus Nitrosopumilus maritimus, was recently isolated into pure culture, showing for the first time nitrification among the domain Archaea. Like known bacterial ammonia oxidizers, N. maritmus converts aerobically ammonia to nitrite and fixes inorganic carbon for biosynthesis. The first aim of this proposal is to understand the biochemical pathways involved in the chemolithoautotrophic life of the only existing pure culture. The studies will be performed in close collaboration with David Stahl at the University of Washington and facilitated by the ongoing whole genome analysis at the Joint Genome Institute. Our part will be the biochemical and microbiological analysis of enzymes involved in the carbon metabolism and the approval of hypothetical pathways. Tracer experiments will be performed to determine if N. maritimus is either strict autotroph or mixotroph. Determination of the carbon isotope fractionation during growth of N. maritimus will provide information on the carbon fixation pathway. The existence of a modified 3-hydroxypropionate cycle for CO2 fixation derived from environmental genomics of crenarchaeal sponge symbionts will be proven biochemically in N. maritimus. The second goal of the proposed project is to analyze the predicted high abundance and diversity of crenarchaeal ammonia oxidizers in natural environments. Previous analyses of 16S rRNA and amoA genes have indicated a ubiquitous distribution of this group. However, it is unclear whether its members are oxidizing ammonia in vivo. The phylogenetic diversity of both genes suggests the existence of various physiotypes. In our project we will isolate further strains from aquatic and terrestrial environments where the corresponding biomarkers have been detected. The new strains will be characterized physiologically and will help to understand the suggested high impact on the biogeochemical nitrogen and carbon cycles.